Tailgate carts have become very popular with gardeners and landscapers in recent times. These carts are hitched to a truck, and provide a large cargo area for the storage and transportation of mowers, ladders, tillers, edging tools, weed-whackers, and other gardening and landscaping equipment. These tailgate carts have a low center of gravity, making them very stable in transporting heavy gardening tools. A tailgate cart of the kind that is described above, is shown in U.S. Pat. Nos. 6,183,031; and 6,126,223.
One outstanding feature of these tailgate carts is their large access door. The door is extends many feet above the loading bed, and its bottom is pivotally connected to the base of the tailgate frame surrounding the loading bed. To access the equipment disposed in the bed of the cart, the door is pulled down about its pivots, until the top of the door touches the roadway. In this position, the door acts like a loading and unloading ramp. The equipment is then easily rolled off the cart for the next landscaping job.
The door of the cart is very heavy, and is latticed to reduce its weight for handling purposes. Yet, despite being latticed, the tailgate door is not easily lifted, or lowered.
In the aforementioned U.S. Pat. Nos. 6,183,031; and 6,126,223, spring assist devices for opening and closing the door, are illustrated and described. These spring devices are mounted on the top of the frame of the tailgate bed. The springs are free to extend along the frame. A cable or chain is attached to the springs via a pulley, and as the door is lowered and raised, the springs are caused to extend and contract, accordingly. The extension and contraction of the springs act to counterbalance the force of the pivoting door in accordance with Hook's law: F=½ K X2, where F is the force required to lower and lift the door, K is the spring constant in pounds/foot2; and X is the distance traveled by the spring in feet.
One of the major problems with these assist devices is that the spring acts in a linear fashion to counterbalance the weight of the door. The arcuate motion of the door, however, does not provide a good force match for the linear spring movement. The result is that the lowering of the door is difficult in the beginning, owing to low lateral force component of the gravity vector in the upright position. As the door descends, the lateral component of the gravity force vector increases, and its mid-range force is better matched to the spring force. However, as the door nears the ground, the lateral gravity force vector component is generally decreasing. The poorly matching spring force, thus makes it difficult to smoothly place the top of the door against the ground.
The present invention, seeks to provide a more uniform counterbalancing of the spring force vis-à-vis the lateral force component of the gravity vector of the tailgate door.
The current invention has designed the spring assist device in two different counterbalancing parts, i.e. two springs act in seriatim to give a non-linear force that more closely matches the lateral force component of the gravitational pull of the tailgate door.
The invention has two springs of unequal length disposed in seriatim. When the tailgate door is being initially lowered, the smaller, or first spring shoulders most of the load. As the door reaches a 45° angle with respect to the tailgate base, the second, longer spring begins to shoulder the existing force. As the top of the door reaches the floor, both the first and second springs are fully extended to allow the door to be gently placed upon the ground.
In the equation, F=½ K X2, the extension of the springs of different length and/or different spring constant of this invention, furnish a non-linear response to the non-linear lateral force component of the door.
Another way of achieving the same result would be to use these two springs of unequal length in seriatim, but each would have a different spring constant “K”. In other words, the first spring would supply a lower counterbalancing force, than the second spring. In this embodiment, the counterbalancing force pattern would be less in the beginning of the door travel, but greater at the end of the door travel vis-à-vis the first embodiment. In other words, the non-linear response pattern can be molded to more closely fit the force vectors of the tailgate door.